(1) Field of the Invention
The present invention relates to a method and a device for optimizing the utilization of an engine, in particular an engine of an aircraft of the rotorcraft type.
(2) Description of Related Art
Most currently-built rotorcraft are fitted with one or two free turbine engines. Power is then taken from a low pressure turbine referred to as a “free” turbine, which low pressure turbine is mechanically independent of the assembly of the engine that comprises the compressor and the high pressure stage, and in particular a high pressure turbine. The free turbine of a turbine engine generally operates at a speed of rotation lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reduction gearbox is needed in the connection with the main rotor of the rotorcraft since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is the main gearbox.
Thermal limitations of a turbine engine, and torque limitations of a main gearbox serve to define a performance envelope covering two normal utilization ratings for a turbine engine arranged on a single or twin-engined rotorcraft:
takeoff rating corresponding to a torque level for the gearbox and heating for the turbine engine that can be accepted for a limited length of time without significant degradation: this is the maximum takeoff power, written PMD by the person skilled in the art, that is usable for a continuous duration of five minutes for example and over an accumulated duration of thirty minutes in a single mission; and
the maximum continuous rating during which, at no time, are the capabilities of the gearbox or the capabilities that result from the maximum acceptable continuous heating of the high pressure blades of the first stage of the turbine exceeded: this is the maximum continuous power, referred to as PMC by the person skilled in the art, and it can be used without any time limit, corresponding to about 90% of the PMD.
On a twin-engined rotorcraft, the performance envelope also covers emergency contingency ratings that are used only when only one of the two turbine engines has failed:
a first emergency rating during which the capabilities of the gearbox concerning its inlet stages and the thermal capabilities of the turbine engine are used to the maximum: this is referred to as super contingency rating and it is equal to about 112% to 120% of PMD, being usable for a continuous duration of thirty consecutive seconds at the most, for example, with this being possible three times during a flight, being referred to as PSU where OEI stands for “one engine inoperative”. If PSU is used, then it is necessary to remove and overhaul the turbine engine;
a second emergency rating during which the capabilities of the gearbox concerning its inlet stages and the capabilities of the turbine engine are used very largely: this is referred to as maximum emergency power and is equal to about 105% to 110% of PMD, and is referred to as PMU or OEI2′ by the person skilled in the art since it is usable for two consecutive minutes, for example; and
a third emergency rating during which the capabilities of the gearbox concerning its inlet stages and the thermal capabilities of the turbine engine are used without damaging them: this is referred to as intermediate emergency power and is equal to PMD, being referred to by the person skilled in the art as continuous OEI or PIU or OEI2, since it can be used continuously for the remainder of the flight after failure of a turbine engine.
Consequently, the thermal and mechanical constraints and above all the phenomenon of turbine blade creep give rise to degradation of the turbine engine to a greater or lesser extent depending on the rating. In order to guarantee safe flight and obtain good performance, it is therefore essential to determine the maximum acceptable level of damage for a turbine engine.
Consequently, an overall utilization potential is evaluated for the turbine engine. Specifically, this amounts to defining a maximum number of flying hours known as time between overhauls (TBO), that the turbine engine is capable of performing since its most recent overhaul or its first use, depending on circumstances. Once TBO has been reached, the turbine engine is removed and then overhauled.
In the text below, and for convenience, the term “most recent overhaul of the turbine engine” is used to designate either the first use of the turbine engine or in fact the most recent overhaul thereof.
Furthermore, in order to obtain flying authorization in a given country for a rotorcraft, it is required that the performance envelope and the TBO of the turbine engine(s) of the rotorcraft to be certified by the official services in the country in question for a precise spectrum of utilizations. Such authorization is thus given only after complete certification tests have been completed, which are expensive.
Since these complete certification tests of a turbine engine are performed in order to justify a performance envelope associated with a TBO, it is not possible to use the turbine engine in some other performance envelope that differs from the initially authorized performance envelope without again performing complete certification tests, which are very expensive.
In addition, when the engine develops an intermediate power between the maximum continuous power PMC and the maximum takeoff power PMD, this intermediate power is subjected to the same limitation as the maximum takeoff power.
Likewise, when the engine is developing power lying between the intermediate emergency power and the maximum emergency power, that power is processed in the same manner as the maximum emergency power, being subjected to the same limitation as said maximum emergency power.
Under such circumstances, operation of the turbine engine would appear not to be optimized because of the discrete staging of engine ratings.
The state of the technical prior art includes document FR 2 878 288 in which it is proposed to modify the maximum number of flying hours in order to optimize use of the engine.
Furthermore, document FR 2 888 287 makes it possible to define a performance envelope that is an alternative to the initial performance envelope of the engine.